Apple's OperatingSystem has been essentially inseparable from its type of WindowingSystems since the introduction of the Macintosh computer in the 1980's. Apple's MacOsx windowing system (Quartz for the accelerated graphics, Aqua for the look-and-feel) descends from the WindowingSystems of the NeXT computers (DisplayPostscript); now the underlying model is based on PDF.

Microsoft has been working on Windows, its example of WindowingSystems for several years now. Until NT, it always seemed to be grafted on top of it's OperatingSystem, now they seem better integrated. This is the windowing system used on most personal computers these days.

PlanNineFromBellLabs had EightAndaHalf? 8&1/2; and now rio which use the intrinsic protocols for file access: Styx. InfernoOs uses the same protocol, but has a different windowing system -- the fact that both were created by the same hands is quite evident in their interoperability.